One of the most important quality requirements of all paper and board grades is uniformity of the structure both on the micro scale and on the macro scale. The structure of paper, in particular of printing paper, must also be symmetric. The good printing properties required from printing paper mean equal good smoothness, evenness, and certain absorption properties of both faces of the web from which the paper will be produced. The properties of paper, such as the symmetry of surface roughness and density, are affected to a considerable extent by the operation of the press section of the paper machine in which the web is produced, which operation also has a decisive significance for the uniformity of the profiles of the paper in the cross direction and in the machine direction.
Increased running speeds of paper machines create new problems to be solved, which problems are mostly related to the runnability of the machine. Currently, running speeds of up to about 1500 meters per minute are employed. At these running speeds, so-called closed press sections, which comprise a compact combination of press rolls fitted around a smooth-faced center roll, usually operate satisfactorily. As examples of such press sections, reference should be made to the assignee's "Sym-Press II".TM. and "Sym-Press O".TM. press sections.
From the point of view of energy economy, dewatering taking place by pressing is preferable to dewatering taking place by evaporation. For this reason, attempts should be made to remove a maximum amount of water out of the paper web by pressing in order that the proportion of water to be removed by evaporation can be made as little as possible. Increased running speeds of paper machines, however, create new, so far unsolved problems expressly for the dewatering taking place by pressing, because the press impulse cannot be increased sufficiently by the means known in the prior art, above all because at high speeds the nip times remain inadequately short and, on the other hand, the peak pressure of pressing cannot be increased beyond a certain limit without destruction of the structure of the web.
In the prior art press sections, the single-felt last press nip tends to produce a poor symmetry of roughness, in particular with fine paper and with LWC and MWC base paper. The problem is manifested with particular emphasis when the press impulse is high, as is the case with an extended-nip press in the last press position. For example, with MWC base paper, with the assignee's test paper machine, when non-calendered, for top-face/bottom-face Bendtsen roughness the value 0.52 was obtained, when the press load was about 800 kN per meter in a "Sym-Belt S".TM. press, the length of the press shoe was about 152 mm, and the smooth press roll was in the upper position of the single-felt press nip. The high asymmetry of roughness constitutes a limitation for the extent of press load, for the dry solids content that can be achieved, and for the wet strength.
A typical environment of application of the present invention, to which environment the present invention is, however, not restricted, is represented by the assignee's Finnish Patent Application 905798 (filed Nov. 23, 1990) and by corresponding U.S. patent application Ser. Nos. 07/795,043 and 08/026,851, the later of which matured into U.S. Pat. No. 5,389,205, the specification of which is hereby incorporated by reference herein.
It is known from the prior art to employ so-called equalizing presses in connection with various press sections, including extended-nip press sections. By means of the equalizing presses, attempts are made to equalize the above asymmetry of roughness. With respect to these prior-art equalizing presses, reference is made, for example, to the assignee's Finnish Patent No. 64,823, to the published German Patent Application No. DE 4,321,406 A1 of Messrs. J. M. Voith GmbH, and to the German Utility Model G 9,206,340.3 of Messrs. Sulzer-Escher Wyss GmbH. By means of the equalizing presses known from the papers mentioned above, it has, however, not been possible to solve the problems related to asymmetry of roughness in a satisfactory way, in particular not in connection with a supported transfer of the web. Of the cited papers mentioned above, the German Utility Model is most closely related to the present invention, in particular the embodiment illustrated in FIG. 12 in that document. In the equalizing press illustrated in FIG. 12 therein, the lower press roll 11 in the equalizing press 5/11 curves the transfer belt 12 and the web over a considerably large angle, and moreover, in connection with the same lower press roll 11, a web transfer nip has been formed by means of a suction roll. Thus, in that construction, it is impossible to make use of differences in speed, by whose means it would be possible to tighten the web after the equalizing press 5/11 so as to eliminate the effects of elongation of the web taking place in the equalizing press. Moreover, in that construction, the abrupt angle of change in direction in a sensitive area directly after the equalizing press restricts the speed of operation of the press.